1. Field of the Invention
This invention relates to a continuous vacuum treating apparatus of an air-to-air system for continuously subjecting workpieces of soft material, such as plastic moldings, to surface treatment as with plasma in a vacuum treating chamber, wherein the plastic moldings refer to plastic films, plastic sheets, sheathed wires, etc.
2. Description of the Prior Art
In one type of continuous vacuum treating apparatus of the air-to-air system known in the art, a plurality of roll seal boxes composed of seal rolls of metal are arranged on the inlet side and the outlet side of a vacuum treating chamber, and a workpiece of hard material, such as a metal tape, is conveyed from the atmosphere into the vacuum treating chamber through the seal roll boxes to apply a coat of metal to the surface of the workpiece as by vaporization deposition in a vacuum and then the workpiece thus treated is returned to the atmosphere through auxiliary vacuum chambers. Such continuous vacuum treating apparatus is disclosed in U.S. Pat. No. 3,367,667, for example.
We have made a proposal to provide an apparatus for continuously subjecting workpieces of soft material, such as plastic sheets, by an air-to-air system in a vacuum chamber to surface treatment as with plasma. This apparatus includes a plurality of auxiliary vacuum chambers located both anterior and posterior to the vacuum treating chamber with each auxiliary vacuum chamber being composed primarily of a pair of seal rolls.
In the apparatus referred to hereinabove, an airtight seal is satisfactorily provided to the auxiliary vacuum chambers. However, problems have been raised with regard to obtaining a compact overall size in a continuous vacuum treating apparatus and economizing on power.
More specifically, in order that the workpiece may be conveyed in good condition, it is necessary that an axial deflection of the pair of seal rolls constituting each auxiliary vacuum chamber caused by pressure differential be kept below a predetermined value (50 .mu.m for example). However, as subsequently to be described, a rise in the level of vacuum in the vacuum treating chamber can best be achieved by raising the level of vacuum in the auxiliary vacuum chamber located closest to the atmosphere. In this case, the differential pressure applied to the seal rolls is much smaller in the auxiliary vacuum chambers on the vacuum treating chamber side than in the auxiliary vacuum chamber closest to the atmosphere. Thus, in view of the need to keep the axial deflection of the seal rolls caused by pressure differential below the predetermined value, it is necessary to increase the diameter of the seal rolls in the auxiliary vacuum chamber closest to the atmosphere. It has hitherto been usual practice in the prior art to use the seal rolls of the same diameter for the auxiliary vacuum chamber located closest to the atmosphere and the auxiliary vacuum chambers disposed on the vacuum treating chamber side.
Owing to this constructional arrangement, leaks through side pieces on opposite end faces of the seal rolls have increased in volume as the diameter of the seal rolls increases, and this has inevitably increased the capacity of the vacuum pumps and the size of the auxiliary vacuum chambers. Also, it has become necessary to increase the power of a system for driving each pair of seal rolls.